The present invention relates to a method and apparatus for looping a coating structure on bead cores in motor-vehicle tires.
It is known that incorporated in vehicle tires, at the so-called "beads" defined along the inner peripheral edges of the tire, are respective annular metal elements, circumferentially inextensible, currently referred to as "bead cores"; said bead cores perform a dual function, that of suitably stiffening the beads themselves for ensuring their stable connection with the rim on which the tire is mounted, and that of providing a steady anchoring to the carcass plies usually arranged in the tire itself.
In a particular embodiment, especially used with tubeless tires for heavy duty motor transport, said bead cores are formed with a plurality of bare metal wire coils having a right polygonal, preferably hexagonal section, disposed axially and radially close to each other so as to constitute an annular element having its radially inner surface inclined at 15.degree. relative to its own axis: a plurality of clips, usually made of a metal material, distributed along the peripheral extension of said element and closed at the right section of same, keeps the element compact and its sectional shape constant during the tire manufacturing process.
The foregoing being stated, it is to be pointed out that during the tire manufacturing process suitable expedients are to be adopted so that a perfect adhesion of the bead core to the other elastomer components of the tire is ensured. The presence of detachment areas and air bubbles entrapped between the bead core and elastomer material disposed around said bead core would be in fact very detrimental to the structure integrity of the tire in use.
To this end, a coating structure made of elastomer material is provided to be applied to the bead core, which coating is adapted to be steadily anchored to the bare metal wires forming the bead core itself and to provide an appropriate attachment surface for the elastomer material components to be further assembled therewith.
To this end, of the great number of known structures, the structure described in U.S. Pat. No. 5,261,979 issued Nov. 16, 1993 in the name of the present assignee has proved to be of particular interest. It consists of a layer of elastomeric material directly wrapped on the bead core itself and a rubberized fabric ribbon provided with reinforcement cords of heat-shrinkable material wrapped around the elastomeric material layer so that said cords are disposed in a direction transverse to the longitudinal extension of the bead core.
In fact, during the tire vulcanization step, an excellent bonding is achieved between the elastomeric material and the bead core, due to the favorable compression action produced on the layer material as a result of the shrinkage of the rubberized fabric cords.
According to the above teachings, the coating structure that can be conveniently prepared separately by assembling the elastomeric material layer and rubberized fabric ribbon, so as to form a strip of appropriate width cut to size, is preferably enfolded loop-wise around the bead core, so that it integrally embraces the cross sectional profile of the bead core itself and locked in said position by overlapping of the longitudinal edges thereof.
This process is particularly convenient when bead cores of the described type are involved, in that it easily overcomes the difficulty embodied by the presence of clips along the peripheral extension of the bead core, which clips give rise to the discontinuity of the surface to be coated making it difficult to use other known methods and machines that bring about a serious negative impact on the final result of the operation.
Unfortunately this looping operation which is easy to do when bead cores of rubberized metal wire are involved, has proved to be a source of several drawbacks, both during its execution and as regards the quality of the finished product, when put into practice according to the known art with bead cores of the pack type, made of bare metal wires as previously described.
This is due to the fact that, with these bead cores, the stable positioning of the ribbon-like structures enfolded on the bead core exclusively depends on the mutual adhesion between the side edges of the coating structure itself, suitably overlapped and pressed against each other by said enfolding operation.
In fact, the adhesion of the raw elastomer material on the bare metal (that is not rubberized) is rather weak and at all events insufficient to ensure the temporary attachment of the first side flap of the coating structure to the bead core during the enfolding or looping operation, above all in the case in which said structure comprises nylon fibers or the like tending to resiliently take on their original conformation again after their enfolding about the bead core section.
In this case, due to the weak adhesiveness between the metal forming the bead core and the elastomeric material forming the coating, the elastic shrinkage of the material can cause separations and slidings of the first flap on the bead core, before said first flap is sealed to the second flap: the result is a loop which is not tensioned and can incorporate air bubbles and where the overlapping width between the two flaps varies along the longitudinal extension of the bead core, which rather often also gives rise to openings proper at the junction point between the two flaps, thereby making the shrinkage action developed by the cords during the tire vulcanization inefficient.
The problem does not seem to be resolvable with the teachings of U.S. Pat. No. 4,450,025, according to which the two flaps are first sealed to each other, at their surfaces axially internal to the loop and disposed in a diametrical plane of the bead core, then the sealed portion of the coating is folded back against the bead core surface.
A drawback present in this solution is, among other things, the fact that the overlapped flaps exhibit an unacceptable thickness at the junction, as compared to the quality standards presently required from the tires.